Micromechanical devices with oscillation systems are used both as micromechanical sensors and micromechanical actuators. The oscillation system consisting of an oscillation body and an elastic suspension exhibits a natural or resonant frequency. In many applications, the resonant frequency of the oscillation system must be in accordance with a fixedly predetermined frequency so as to achieve, utilizing the resonance enhancement, sufficient sensitivity such as in the case of a sensor, and a sufficient oscillation amplitude such as in the case of an actuator. Examples of such micromechanical devices with an oscillation system are clock generators in clocks or deflecting mirror such as scanner mirrors used for data protection. In the last-mentioned scanner mirrors, for example, the data frequency or modulation frequency and the oscillation frequency must be in a fixedly predetermined ratio to each other. A further example for an application, where a nominal frequency is given, is present when a pair of a sensor or actuator that are basically identical in construction are to be synchronized to each other.
In order to keep the power to be consumed for the oscillation generation low, oscillation systems of such devices are generally of a relatively high Q, with the result that the resonance curve is narrow and that, if the desired oscillation amplitude is adhered to, there will be very little tolerance in the excitation frequency.
The reasons for a deviation of the resonant frequency of the oscillation system of a micromechanical device from a nominal resonant frequency are extremely manifold and may coarsely be divided into two groups, namely such leading to a constant resonant frequency deviation or resonant frequency offset in spite of identical and constant environmental conditions and caused, for example, by production or fabrication variations/tolerances, and such that are subject to temporal changes or caused, for example, by variations in the environmental conditions. In the following, for the constant, for example, fabrication related deviation of the actual resonant frequency of a micromechanical device from its nominal resonant frequency, the term “resonant-frequency deviation” will be used, whereas for the frequency deviations subject to temporal changes during operation or lifetime, the term “resonant-frequency variation” will be used.
The term “resonant-frequency deviation” therefore also includes the mismatch in the resonant frequency of devices that are basically identical in construction, which occurs in spite of identical and constant environmental conditions. The reason for this lies in variations of frequency-determining material parameters such as elastic constants, density, etc., and statistical or systematical deviations in the dimensions of spring and mass or gaps having a dampening effect due to tolerances relating to adjustment, structuring and layer generation in the fabrication of the micromechanical devices.
The term resonant-frequency variation, in contrast, is meant to describe the variation of the resonant frequency of the oscillation system of a micromechanical device caused by, for example, variations in the environmental conditions such as variations in pressure or temperature. However, resonant-frequency variations may also be the result of different degrees of absorption of different gas molecules, humidity and the like on the oscillation system or of temporal changes of the material parameters.
The known measures for adjusting the resonant frequency of the oscillation system of a micromechanical device to a nominal resonant frequency may also be divided into two strategy types, namely a strategy, according to which in quasi one of the last fabrication steps, non-reversible changes are performed on the micromechanical devices for matching the resonant frequency of the oscillation system, and a strategy, according to which the resonant frequency of the oscillation system is corrected to the nominal resonant frequency during operation, such as readjusted via a control loop. The first strategy is, of course, suitable for the compensation of permanent resonant-frequency deviations only and in some applications necessitating compensation of the resonant-frequency variations as well cannot substitute a resonant-frequency correction during operation.
There are several approaches for regulating the resonant frequency during operation. U.S. Pat. No. 6,331,909 and U.S. Pat. No. 6,285,489 describe a resonant-frequency regulation where, for altering the resonant frequency, the ambient pressure is varied, whereby the effective mass of the element moved or the oscillation body changes due to the gas loading, whereby the resonant frequency of the spring-mass system also changes. The necessitated apparatus and control circuit are, however, relatively complex. Furthermore, an embodiment is described, wherein the spring of the spring-mass system is loaded with a gas-absorbing material that, during absorption, changes the material properties and therefore the frequency. Here, too, the disadvantage is the relatively high complexity. Moreover, one may assume that, as a result of the limitations in the choice of the materials of the gas-absorbing type as such available for the spring, the Q of the system will be degraded and may not be optimal.
In U.S. Pat. No. 6,256,131 and U.S. Pat. No. 6,285,489, a torsion oscillation system is described, wherein a portion of the rotating mass may be shifted away from the torsion axis or towards the torsion axis, respectively, by means of electrostatic forces. This changes the moment of inertia and, in turn, the resonant frequency. Although this procedure allows regulating the resonant frequency, larger-scale deviations cannot be corrected due to the generally small translation paths of the movable mass. Additional electrical lines as a result of the elastic suspension or torsion springs or on the torsion springs make this embodiment complex, resulting in increased spatial requirements on the mirror plate. This also increases the dynamic deformation.
In another embodiment, a micromechanical device with a matchable resonant frequency is described according to EP 1613969 A1. With the help of geometrical structures, such as ribs, which may systematically be broken by external influences, the effective length and therefore the rigidity of micromechanical spring elements is influenced in an irreversible and discrete manner. During operation, a virtual spring-constant increase or reduction may be achieved by applying a voltage difference between the oscillation body and suitably arranged stationary electrodes.